This invention relates to gas turbine engines having high turbine inlet temperatures and ceramic rotor blades. More specifically, the invention relates to construction of rotors that carry ceramic blades.
In order to understand the invention, it is helpful to consider operation and structure of conventional gas turbines. FIG. 1 is a schematic of a basic idealized gas turbine system useful for explaining the limitations of conventional gas turbine engines. In the engine schematic of FIG. 1, intake air 10 enters the compressor 11. Compressor 11 increases the pressure of air 10 with little heat loss and consequent rise in temperature (substantially adiabatic) and outputs compressed air 18 to a combustion chamber 12. In combustion chamber 12, compressed air 18 mixes with fuel 13 and the resulting air/fuel mixture ignited to raise the temperature of the air under constant pressure conditions. Fast moving hot gas 14 exiting combustion chamber 12 feeds into turbine 15, expanding and imparting mechanical forces to rotor blades located within turbine 15. The aerodynamic lift, drag, and other forces that deflect the moving gas stream 14 are collectively called gas dynamic forces. These gas dynamic forces deliver mechanical energy to the rotor blades therefore rotating a shaft 16 that drives compressor 11 and performs other useful work. The expanded and cooled gas is finally expelled as exhaust 17.
The thermal efficiency of the gas turbine measures the amount of work produced from a given quantity of fuel. Thermal efficiency is a function of the magnitude of the pressure and temperature drop of gas 14 across turbine 15. Thus, for a constant pressure and temperature of exhaust gas 17, thermal efficiency improves in proportion to increases in the temperature and pressure of turbine inlet gas 14.
In conventional turbine designs, turbine inlet temperatures are limited to approximately 2200.degree. F.-2600.degree. F. by available rotor-blade materials and blade cooling technologies. Temperatures within combustion chamber 12, however, can be more than 3800.degree. F. These ultra-hot combustion gases must be cooled by diluting them with excess compressed air 18 introduced in combustion chamber 12 to lower the temperature of inlet gas 14 to the minimum allowable temperature of 2200.degree. F.-2600.degree. F. The need to cool inlet gas 14 thus limits the thermal efficiency of conventional turbines.
Ceramic turbine blades have been proposed as a means of expanding the thermal operating envelope of the turbine. Ceramic materials retain structural strength at temperatures in excess of the current maximum allowable inlet temperatures and may potentially eliminate the present requirement to cool the combustion gases. Ceramic materials, however, lack the necessary strength and ductility to withstand the mechanical loads of the turbine environment. In particular, monolithic ceramic blades, similar in construction to conventional metal blades, are unable to meet tensile, fatigue, thermal shock, and ductility requirements even for short duration or partial load turbine operations.
The improvements in turbine engine efficiency possible by incorporating ceramics as a blade material are presently thus more theoretical than practicable. Turbine engine efficiency is therefore limited by the thermal and mechanical properties of existing blade materials and turbine construction.